Subject: Re: [PATCH] ebtables: fix race condition in frame_filter_net_init()

From: Florian Westphal <[email-masked]>
Date: Tue, 26 Sep 2017 14:42:11 +0200

Artem Savkov <[email-masked]> wrote:
> It is possible for ebt_in_hook to be triggered before ebt_table is assigned
> resulting in a NULL-pointer dereference. Make sure hooks are
> registered as the last step.

Right, thanks for the patch.

> --- a/net/bridge/netfilter/ebtable_broute.c
> +++ b/net/bridge/netfilter/ebtable_broute.c
> @@ -65,7 +65,7 @@ static int ebt_broute(struct sk_buff *skb)
>  
>  static int __net_init broute_net_init(struct net *net)
>  {
> -	net->xt.broute_table = ebt_register_table(net, &broute_table, NULL);
> +	net->xt.broute_table = ebt_register_table(net, &broute_table);

I wonder if it makes more sense to model this like the iptables version,
i.e. pass net->xt.table_name as last arg to ebt_register_table ...

> +int ebt_register_hooks(struct net *net, struct ebt_table *table,
> +		      const struct nf_hook_ops *ops)
> +{
> +	int ret = nf_register_net_hooks(net, ops, hweight32(table->valid_hooks));
> +
> +	if (ret)
> +		__ebt_unregister_table(net, table);
> +
> +	return ret;
> +}

... because this looks strange (unregister of table/not-so-obvious error
unwinding ...)

> @@ -1252,15 +1262,6 @@ ebt_register_table(struct net *net, const struct ebt_table *input_table,
>  	list_add(&table->list, &net->xt.tables[NFPROTO_BRIDGE]);
>  	mutex_unlock(&ebt_mutex);

... here one could then assign the net->xt.table_X pointer, and then do
the hook registration right after.

However i have no strong opinion here.